Measurement apparatus and a method for determining a substrate grid

ABSTRACT

A measurement apparatus and method for determining a substrate grid describing a deformation of a substrate prior to exposure of the substrate in a lithographic apparatus configured to fabricate one or more features on the substrate. Position data for a plurality of first features and/or a plurality of second features on the substrate is obtained. Asymmetry data for at least a feature of the plurality of first features and/or the plurality of second features is obtained. The substrate grid based on the position data and the asymmetry data is determined. The substrate grid and asymmetry data are passed to the lithographic apparatus for controlling at least part of an exposure process to fabricate one or more features on the substrate.

This application claims the benefit of priority of U.S. provisionalapplication No. 62/773,576, filed Nov. 30, 2018, and of European patentapplication no. 18154053, filed Jan. 30, 2018. Each of the foregoingapplications is incorporated herein in its entirety by reference.

FIELD

The present description relates to a measurement apparatus and a methodfor determining a substrate grid.

BACKGROUND

A lithographic apparatus is a machine constructed to apply a desiredpattern onto a substrate. A lithographic apparatus can be used, forexample, in the manufacture of devices such as integrated circuits(ICs). A lithographic apparatus may, for example, project a pattern(also often referred to as “design layout” or “design”) at a patterningdevice (e.g., a mask) onto a layer of radiation-sensitive material(resist) provided on a substrate (e.g., a wafer).

To project a pattern on a substrate a lithographic apparatus may useelectromagnetic radiation. The wavelength of this radiation determinesthe minimum size of features which can be formed on the substrate.Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nmand 13.5 nm. A lithographic apparatus, which uses extreme ultraviolet(EUV) radiation, having a wavelength within the range 4-20 nm, forexample 6.7 nm or 13.5 nm, may be used to form smaller features on asubstrate than a lithographic apparatus which uses, for example,radiation with a wavelength of 193 nm.

Low-k₁ lithography may be used to process features with dimensionssmaller than the classical resolution limit of a lithographic apparatus.In such processing, the resolution formula may be expressed asCD=k₁×λ/NA, where λ is the wavelength of radiation employed, NA is thenumerical aperture of the projection optics in the lithographicapparatus, CD is the “critical dimension” (generally the smallestfeature size printed, but in this case half-pitch) and k₁ is anempirical resolution factor. In general, the smaller k₁ the moredifficult it becomes to reproduce the pattern on the substrate thatresembles the shape and dimensions planned by a circuit designer inorder to achieve particular electrical functionality and performance. Toovercome these difficulties, sophisticated fine-tuning steps may beapplied to the lithographic projection apparatus and/or design layout.These include, for example, but are not limited to, optimization of NA,customized illumination schemes, use of phase shifting patterningdevices, various optimization of the design layout such as opticalproximity correction (OPC) in the design layout, or other methodsgenerally defined as “resolution enhancement techniques” (RET).Additionally or alternatively, tight control loops for controlling astability of the lithographic apparatus may be used to improvereproduction of a pattern at low k1.

SUMMARY

It is known to measure the position of several substrate alignment marksin order to produce a substrate grid which provides a description of adeformation of a substrate prior to exposure of the substrate in alithographic apparatus.

Embodiments described herein may have use in an EUV lithographicapparatus. Embodiments may have use in a deep ultraviolet (DUV)lithographic apparatus and/or another form of tool.

In an aspect, there is provided a method for determining a substrategrid describing a deformation of a substrate prior to exposure of thesubstrate in a lithographic apparatus configured to fabricate one ormore features on the substrate, the method comprising: obtainingposition data for a plurality of first features and/or a plurality ofsecond features on the substrate; obtaining asymmetry data for at leasta feature of the plurality of first features and/or the plurality ofsecond features; determining the substrate grid based on the positiondata; and passing the substrate grid and asymmetry data to thelithographic apparatus for controlling at least part of an exposureprocess to fabricate one or more features on the substrate.

In an aspect, there is provided a measurement apparatus configured fordetermining a substrate grid describing a deformation of a substrateprior to exposure of the substrate in a lithographic apparatusconfigured to fabricate one or more features on the substrate, themeasurement apparatus comprising: an optical system configured to obtainposition data for a plurality of first features and/or a plurality ofsecond features on the substrate, wherein the optical system is furtherconfigured to obtain asymmetry data for at least a feature of theplurality of first features and/or the plurality of second features, andwherein the measurement apparatus is configured to determine thesubstrate grid based on the position data or both the position andasymmetry data, and pass the substrate grid and asymmetry data to thelithographic apparatus for controlling at least part of an exposureprocess to fabricate one or more features on the substrate.

In an aspect, there is provided a method for determining a value for aprocess parameter measurement error obtained from measurement of asubstrate subject to a manufacturing process and comprising a targethaving a process distortion, the process parameter measurement errorbeing a result of the process distortion, the method comprising:obtaining alignment asymmetry data describing asymmetry in one or morealignment marks used for aligning the substrate; obtaining a modelcorrelating alignment asymmetry data to the process parametermeasurement error; and using the alignment asymmetry data and the modelto obtain the value of the process parameter measurement error.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings, in which:

FIG. 1 depicts a schematic overview of a lithographic apparatus;

FIG. 2 depicts a schematic overview of a lithographic cell;

FIG. 3 depicts a schematic representation of a lithography techniqueinvolving a cooperation between three technologies to optimizesemiconductor manufacturing;

FIG. 4 depicts a schematic representation of a measurement apparatus fordetermining position and asymmetry of features;

FIG. 5 depicts a schematic representation of an alignment and levellingsensor and an exposure apparatus in a lithographic apparatus;

FIG. 6 depicts a schematic representation of a metrology apparatus formeasuring overlay;

FIG. 7 depicts a flow diagram of the method for determining a substrategrid, alignment in the lithographic apparatus and measurement ofoverlay; and

FIG. 8 depicts a flow diagram of a method for determining a value for anoverlay measurement error according to an embodiment of the invention.

DETAILED DESCRIPTION

In the present document, the terms “radiation” and “beam” are used toencompass all types of electromagnetic radiation, including ultravioletradiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) andEUV (extreme ultra-violet radiation, e.g. having a wavelength in therange of about 5-100 nm).

The term “reticle”, “mask” or “patterning device” as employed in thistext may be broadly interpreted as referring to a generic patterningdevice that can be used to endow an incoming radiation beam with apatterned cross-section, corresponding to a pattern that is to becreated in a target portion of the substrate. The term “light valve” canalso be used in this context. Besides the classic mask (transmissive orreflective, binary, phase-shifting, hybrid, etc.), examples of othersuch patterning devices include a programmable mirror array and aprogrammable LCD array.

FIG. 1 schematically depicts a lithographic apparatus LA. Thelithographic apparatus LA includes an illumination system (also referredto as illuminator) IL configured to condition a radiation beam B (e.g.,UV radiation, DUV radiation or EUV radiation), a support (e.g., a masktable) MT constructed to support a patterning device (e.g., a mask) MAand connected to a first positioner PM configured to accurately positionthe patterning device MA in accordance with certain parameters, asubstrate support (e.g., a wafer table) WT constructed to hold asubstrate (e.g., a resist coated wafer) W and connected to a secondpositioner PW configured to accurately position the substrate support inaccordance with certain parameters, and a projection system (e.g., arefractive projection lens system) PS configured to project a patternimparted to the radiation beam B by patterning device MA onto a targetportion C (e.g., comprising one or more dies) of the substrate W.

In operation, the illumination system IL receives a radiation beam froma radiation source SO, e.g. via a beam delivery system BD. Theillumination system IL may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic,electrostatic, and/or other types of optical components, or anycombination thereof, for directing, shaping, and/or controllingradiation. The illuminator IL may be used to condition the radiationbeam B to have a desired spatial and angular intensity distribution inits cross section at a plane of the patterning device MA.

The term “projection system” PS used herein should be broadlyinterpreted as encompassing various types of projection system,including refractive, reflective, catadioptric, anamorphic, magnetic,electromagnetic and/or electrostatic optical systems, or any combinationthereof, as appropriate for the exposure radiation being used, and/orfor other factors such as the use of an immersion liquid or the use of avacuum. Any use of the term “projection lens” herein may be consideredas synonymous with the more general term “projection system” PS.

The lithographic apparatus LA (or scanner) may be of a type wherein atleast a portion of the substrate may be covered by a liquid having arelatively high refractive index, e.g., water, so as to fill a spacebetween the projection system PS and the substrate W—which is alsoreferred to as immersion lithography. More information on immersiontechniques is given in U.S. Pat. No. 6,952,253, which is incorporatedherein in its entirety by reference.

The lithographic apparatus LA may also be of a type having two or moresubstrate supports WT (also named “dual stage”). In such “multiplestage” machine, the substrate supports WT may be used in parallel,and/or steps in preparation of a subsequent exposure of the substrate Wmay be carried out on the substrate W located on one of the substratesupport WT while another substrate W on the other substrate support WTis being used for exposing a pattern on the other substrate W.

In addition to the substrate support WT, the lithographic apparatus LAmay comprise a measurement stage. The measurement stage is arranged tohold a sensor and/or a cleaning device. The sensor may be arranged tomeasure a property of the projection system PS or a property of theradiation beam B. The measurement stage may hold multiple sensors. Thecleaning device may be arranged to clean part of the lithographicapparatus, for example a part of the projection system PS or a part of asystem that provides the immersion liquid. The measurement stage maymove beneath the projection system PS when the substrate support WT isaway from the projection system PS.

In operation, the radiation beam B is incident on the patterning device,e.g. mask, MA which is held on the support MT, and is patterned by thepattern (design layout) present on patterning device MA. Havingtraversed the patterning device MA, the radiation beam B passes throughthe projection system PS, which focuses the beam onto a target portion Cof the substrate W. With the aid of the second positioner PW and aposition measurement system IF (e.g., LA alignment sensor), thesubstrate support WT can be moved accurately, e.g., so as to positiondifferent target portions C in the path of the radiation beam B at afocused and aligned position. Similarly, the first positioner PM andpossibly another position sensor (which is not explicitly depicted inFIG. 1) may be used to accurately position the patterning device MA withrespect to the path of the radiation beam B. Patterning device MA andsubstrate W may be aligned using patterning device alignment marks M1,M2 and substrate alignment marks P1, P2. Although the substratealignment marks P1, P2 as illustrated occupy dedicated target portions,they may be located in spaces between target portions. Substratealignment marks P1, P2 are known as scribe-lane alignment marks whenthese are located between the target portions C.

As shown in FIG. 2 the lithographic apparatus LA may form part of alithographic cell LC, also sometimes referred to as a lithocell or(litho)cluster, which often also includes apparatus to perform pre- andpost-exposure processes on a substrate W. Conventionally these includeone or more spin coaters SC to deposit resist layers, one or moredevelopers DE to develop exposed resist, one or more chill plates CHand/or one or more bake plates BK, e.g. for conditioning the temperatureof substrates W e.g. for conditioning solvents in the resist layers. Asubstrate handler, or robot, RO picks up substrates W from input/outputports I/O1, I/O2, moves them between the different process apparatus anddelivers the substrates W to the loading bay LB of the lithographicapparatus LA. The devices in the lithocell, which are often alsocollectively referred to as the track, are typically under the controlof a track control unit TCU that in itself may be controlled by asupervisory control system SCS, which may also control the lithographicapparatus LA, e.g. via lithography control unit LACU.

In order for the substrates W exposed by the lithographic apparatus LAto be exposed correctly and consistently, it is desirable to inspectsubstrates to measure a value of one or more properties of patternedstructures, such as overlay error values between subsequent layers, linethicknesses, critical dimension values (CD), etc. For this purpose, oneor more inspection tools (not shown) may be included in the lithocellLC. If an error is detected, an adjustment, for example, may be made toexposures of subsequent substrates or to other processing steps that areto be performed on the substrates W, especially if the inspection isdone before other substrates W of the same batch or lot are still to beexposed or processed.

An inspection apparatus, which may also be referred to as a metrologyapparatus, is used to determine one or more properties of the substratesW, and in particular, how values of one or more properties of differentsubstrates W vary or how values of one or more properties associatedwith different layers of the same substrate W vary from layer to layer.The inspection apparatus may alternatively be constructed to identifydefects on the substrate W and may, for example, be part of thelithocell LC, or may be integrated into the lithographic apparatus LA,or may even be a stand-alone device. The inspection apparatus maymeasure the one or more properties on a latent image (image in a resistlayer after the exposure), or on a semi-latent image (image in a resistlayer after a post-exposure bake step PEB), or on a developed resistimage (in which the exposed or unexposed parts of the resist have beenremoved), or even on an etched image (after a pattern transfer step suchas etching).

Typically the patterning process in a lithographic apparatus LA is asignificant step in the processing and involves high accuracy ofdimensioning and placement of structures on the substrate W. To helpensure this high accuracy, three systems may be combined in a controlenvironment as schematically depicted in FIG. 3. One of these systems isthe lithographic apparatus LA which is (virtually) connected to ametrology tool MET (a second system) and to a computer system CL (athird system). An aim of such a control environment is to optimize thecooperation between these three systems to enhance the overall processwindow and provide tight control loops to help ensure that thepatterning performed by the lithographic apparatus LA stays within aprocess window. The process window defines a range of process parameters(e.g. dose, focus, overlay) within which a specific manufacturingprocess yields a defined result (e.g. a functional semiconductordevice)—typically within which the process parameters in thelithographic process or patterning process are allowed to vary.

The computer system CL may use (part of) the design layout to bepatterned to predict which resolution enhancement techniques to use andto perform computational lithography simulations and calculations todetermine which patterning device (e.g., mask) layout and lithographicapparatus settings achieve a large or largest overall process window ofthe patterning process (depicted in FIG. 3 by the double arrow in thefirst scale SC1). Typically, the resolution enhancement techniques arearranged to match the patterning possibilities of the lithographicapparatus LA. The computer system CL may also be used to detect wherewithin the process window the lithographic apparatus LA is currentlyoperating (e.g. using input from the metrology tool MET) to predictwhether defects may be present due to e.g. sub-optimal processing(depicted in FIG. 3 by the arrow pointing “0” in the second scale SC2).

The metrology tool MET may provide input to the computer system CL toenable accurate simulations and predictions, and may provide feedback tothe lithographic apparatus LA to identify possible drifts, e.g. in acalibration status of the lithographic apparatus LA (depicted in FIG. 3by the multiple arrows in the third scale SC3).

The lithographic apparatus LA is configured to accurately reproduce thepattern onto the substrate. The positions and dimensions of the appliedfeatures should be within certain tolerances. Position errors may occurdue to an overlay error (often referred to as “overlay”). The overlay isthe error in placing a first feature during a first exposure relative toa second feature during a second exposure. The lithographic apparatusaims to reduce or minimize the overlay errors by aligning each substrateaccurately to a reference prior to patterning. This is done by measuringpositions of alignment marks on the substrate using an alignment sensor.More information on the alignment procedure can be found in U.S. PatentApplication Publication No. US 2010-0214550, which is incorporatedherein in its entirety by reference. Pattern dimensioning (CD) errorsmay e.g. occur when the substrate is not positioned correctly withrespect to a focal plane of the lithographic apparatus. A focal positionerror may be associated with un-flatness of a substrate surface. Thelithographic apparatus reduces or minimizes the focal position error bymeasuring the substrate surface topography prior to patterning using alevel sensor. Substrate height corrections are applied during subsequentpatterning to help assure correct imaging (focusing) of the patterningdevice pattern onto the substrate. More information on the level sensorsystem can be found in U.S. Patent Application Publication No. US2007-0085991, which is incorporated herein in its entirety by reference.

Besides the lithographic apparatus LA and the metrology apparatus MET,one or more other processing apparatuses may be used during deviceproduction as well. An etching station (not shown) processes thesubstrates after exposure of the pattern into the resist. The etchstation transfers the pattern from the resist into one or more layersunderlying the resist layer. Typically etching is based on applicationof a plasma medium. One or more local etching characteristics may e.g.be controlled using temperature control of the substrate or directingthe plasma medium using a voltage controlled ring. More information onetching control can be found in PCT Patent Application Publication No.WO 2011/081645 and U.S. Patent Application Publication No. US2006-0016561, each of which is incorporated herein in its entirety byreference.

During the manufacturing of devices (such as ICs) it is desirable thatthe process conditions for processing substrates using one or moreprocessing apparatuses such as the lithographic apparatus or etchingstation remain stable such that values of one or more properties of thefeatures remain within certain control limits. Stability of the processis of particular significance for features of the functional parts ofthe device, the product features. To help ensure stable processing,process control capabilities should be in place. Process controlinvolves monitoring of processing data and implementation of means forprocess correction, e.g. control the processing apparatus based oncharacteristics of the processing data. Process control may be based onperiodic measurement by the metrology apparatus MET, often referred toas “Advanced Process Control” (also referred to as APC). Moreinformation on APC can be found in U.S. Patent Application PublicationNo. US 2012-0008127, which is incorporated herein in its entirety byreference. A typical APC implementation involves periodic measurementson metrology features on the substrates to monitor and correct driftsassociated with one or more processing apparatuses. The metrologyfeatures reflect the response to process variations of the productfeatures. The sensitivity of the metrology features to processvariations may be different compared to the product features. In thatcase a so-called “Metrology To Device” offset (also referred to as MTD)may be determined. To mimic the behavior of product features themetrology targets may incorporate segmented features, assist features orfeatures with a particular geometry and/or dimension. A carefullydesigned metrology target should respond in a similar fashion to processvariations as the product features. More information on metrology targetdesign can be found in PCT Patent Application Publication No. WO2015/101458, which is incorporated herein in its entirety by reference.

The distribution of the locations across the substrate and/or patterningdevice where the metrology targets are present and/or measured is oftenreferred to as the “sampling scheme”. Typically the sampling scheme isselected based on an expected fingerprint of the relevant processparameter(s); areas on the substrate where a process parameter isexpected to fluctuate are typically sampled more densely than areaswhere the process parameter is expected to be constant. Further there isa limit to the number of metrology measurements which may be performedbased on the allowable impact of the metrology measurements on thethroughput (e.g., number of substrates processed per unit time) of thelithographic process. A carefully selected sampling scheme issignificant to accurately controlling the lithographic process withoutaffecting throughput and/or assigning a too large area on the reticle orsubstrate to metrology features. Technology related to optimalpositioning and/or measuring metrology targets is often referred to as“scheme optimization”. More information on scheme optimization can befound in PCT Patent Application Publication Nos. WO 2015/110191 and WO2018/069015, each of which is incorporated herein in its entirety byreference.

Besides metrology measurement data also context data may be used forprocess control. Context data may comprise data relating to one or moreof: the selected processing tools (out of the pool of processingapparatuses), specific characteristics of the processing apparatus, thesettings of the processing apparatus, the design of the patterningdevice pattern and/or measurement data relating to processing conditions(for example substrate geometry). Examples of using context data forprocess control purposes may be found in the PCT Patent ApplicationPublication Nos. WO 2017/140532 and WO 2017/060080, each of which isincorporated herein in its entirety by reference. Context data may beused to control or predict processing in a feed-forward manner in casethe context data relates to process steps performed before the currentlycontrolled process step. Often context data is statistically correlatedto one or more product feature properties. This enables context drivencontrol of one or more processing apparatuses in view of achievingimproved or optimal values of one or more product feature properties.Context data and metrology data may also be combined e.g. to enrichsparse metrology data to an extent that more detailed (dense) databecomes available which is more useful for control and/or diagnosticpurposes. More information on combining context data and metrology datacan be found in PCT Patent Application Publication No. WO 2017/144379,which is incorporated herein in its entirety by reference.

As the monitoring of a process is based on acquisition of data relatedto the process, the data sampling rate (per lot or per substrate) andsampling density depend on the desired level of accuracy of patternreproduction. For low-k1 lithographic processes even small substrate tosubstrate process variations may be significant. The context data and/ormetrology data should then be sufficient to enable process control on aper substrate basis. Additionally when a process variation gives rise tovariations of a characteristic across the substrate, the density of thecontext and/or metrology data should be sufficiently distributed acrossthe substrate. However the time available for metrology (measurements)is limited in view of the desired throughput of the process. As a resultof this limitation, the metrology tool may measure only on selectedsubstrates and/or selected locations across the substrate. Strategies todetermine what substrates should be measured are further described inPCT Patent Application Publication Nos. WO 2018/072980 and WO2018/072962, each of which is incorporated herein in its entirety byreference.

In practice, it may be necessary to derive a denser map of values from asparse set of measurement values relating to a process parameter (acrossa substrate or plurality of substrates). Typically such a dense map ofmeasurement values may be derived from the sparse measurement data inconjunction with a model associated with an expected fingerprint of theprocess parameter. More information on modeling measurement data can befound in PCT Patent Application Publication No. WO 2013/092106 which isincorporated herein in its entirety by reference.

In lithographic processes, it is desirable to make frequent measurementsof the structures created, e.g., for process control and verification.Tools to make such measurement are typically called metrology tools.Different types of metrology tools for making such measurements areknown, including scanning electron microscopes or various forms ofscatterometer metrology tools. Scatterometers are versatile instrumentswhich allow measurements of the parameters of a lithographic process byhaving a sensor in the pupil or a conjugate plane with the pupil of theobjective of the scatterometer, measurements usually referred as pupilbased measurements, or by having the sensor in the image plane or aplane conjugate with the image plane, in which case the measurements areusually referred as image or field based measurements. Suchscatterometers and the associated measurement techniques are furtherdescribed in U.S. Patent Application Publication Nos. US 2010-0328655,US 2011-102753, US 2012-0044470, US 2011-0249244, and US 2011-0026032and in European Patent Application Publication No. EP1628164. Each ofthe foregoing patent application publications is incorporated herein inits entirety by reference. Aforementioned scatterometers may measuregratings using radiation from soft x-ray radiation, extreme ultravioletradiation, visible light to near-IR wavelength range.

In one arrangement, the scatterometer is an angular resolvedscatterometer. In such a scatterometer reconstruction methods may beapplied to the measured signal to reconstruct or calculate a value ofone or more properties of the grating. Such reconstruction may, forexample, result from simulating interaction of scattered radiation witha mathematical model of the target structure and comparing thesimulation results with those of a measurement. Parameters of themathematical model are adjusted until the simulated interaction producesa diffraction pattern similar to that observed from the real target.

In a further arrangement, the scatterometer is a spectroscopicscatterometer. In such a spectroscopic scatterometer, the radiationemitted by a radiation source is directed onto the target and thereflected or scattered radiation from the target is directed to aspectrometer detector, which measures a spectrum (i.e. a measurement ofintensity as a function of wavelength) of the specular reflectedradiation. From this data, the structure or profile of the target givingrise to the detected spectrum may be reconstructed, e.g. by RigorousCoupled Wave Analysis and non-linear regression or by comparison with alibrary of simulated spectra.

The scatterometer may be adapted to measure the overlay of twomisaligned gratings or periodic structures by measuring asymmetry in thereflected spectrum and/or the detection configuration, the asymmetrybeing related to the extent of the overlay. The two (typicallyoverlapping) grating structures may be applied in, for example, twodifferent layers (not necessarily consecutive layers), and may be formedsubstantially at the same position on the substrate. The scatterometermay have a symmetrical detection configuration as described e.g. inEuropean Patent Application Publication No. EP1628164, which isincorporated herein in its entirety by reference, such that anyasymmetry is clearly distinguishable. This provides a straightforwardway to measure misalignment in gratings. Further examples for measuringoverlay error between the two layers containing periodic structures as atarget by measuring through asymmetry of the periodic structures may befound in PCT Patent Application Publication No. WO 2011/012624 and U.S.Patent Application Publication No. US 2016-0161863, each of which isincorporated herein in its entirety by reference.

FIG. 4 shows a measurement apparatus, hereafter referred to as a feedforward metrology cluster (FFMC) 10, which is positioned to takemeasurements from a substrate 12 prior to the substrate 12 being passedinto the lithographic apparatus LA for exposure. The FFMC 10 may bepositioned after the photoresist deposition tool and before thelithographic apparatus LA. The FFMC 10 may be a dual stage system havinga first stage 14 and a second stage 16.

The first stage 14 of the FFMC 10 comprises an alignment sensor 18.Typically, first stage 14 may also comprise a sensor system providingleveling of the substrate with respect to the other sensor systems(e.g., the alignment sensor 18), typically referred to as a levelsensor. Any class of level sensor may be utilized; e.g., a gas (e.g.,air) gauge, an optical level sensor, a stamp based sensor (based onmechanical interaction with the substrate), etc. Typically a levelsensor does not provide a single distance number between the substrateand the other sensors, but it samples the height of the substrate at aplurality of locations in order to determine a height profile of thesubstrate. The alignment sensor 18 is configured to measure positiondata of a plurality of first features, which in this exemplary case arealignment marks. The alignment marks may comprise diffraction gratingshaving a particular pitch. Typically the pitch of the alignment marks issubstantially larger than the wavelength of the radiation used tomeasure the position of the alignment mark and hence the alignmentsensor 18 may have a low numerical aperture (NA), desirably smaller thanor equal to 0.9. In view of the required NA of the alignment sensor tomeasure the position of the first features, these first features arereferred to herein as “low NA alignment marks”.

It might be advantageous for the alignment sensor 18 to be a low NAsensor because alignment and leveling is done on the same substrate, andspace is required between the level sensor and the substrate 12 to carryout the leveling.

To obtain the position data of the plurality of the low NA alignmentmarks, the alignment sensor 18 is scanned across the substrate 12 (e.g.,by moving the substrate 12 relative to the alignment sensor 18).

The second stage 16 of the FFMC 10 comprises one or more, in thisexemplary case three, metrology sensors 20 (i.e. optical systems)configured to measure the position of the plurality of low NA alignmentmarks. The metrology sensor 20 may also measure the asymmetry of afeature of the plurality of low NA alignment marks. In other examples,the second stage 16 may include more or less than three metrologysensors 20.

The metrology sensors 20 may comprise a high numerical aperture (NA)optical system, e.g. having an NA of greater than 0.9. That is, one ormore of the metrology sensors 20 is a high NA sensor. Thus, as shown instep 102 of FIG. 7, position and asymmetry data for the low NA alignmentmarks may be obtained using a high-NA optical system comprising a sensorhaving a NA of greater than for example 0.9.

The metrology sensors 20 may be located relatively close to thesubstrate 12, when compared to the alignment sensor 18, and so can havea relatively large NA.

The metrology sensor 20 may for example measure position data of the lowNA alignment marks by detecting the low NA alignment marks in the imageplane of the metrology sensor 20, and the asymmetry data by viewing thelow NA alignment marks in the pupil plane of the metrology sensor 20.

More particularly, analysis is carried out on radiation scattered fromthe low NA alignment marks. The asymmetry in intensities associated witha diffraction (angular) spectrum of the scattered radiation from the lowNA alignment marks allows the asymmetry of the low NA alignment marks tobe determined. The metrology sensor 20 may be a high NA sensor in orderto be able to detect the diffraction spectrum of the scatteredradiation.

Using multiple (e.g., three) metrology sensors 20 allows for anincreased number of low NA alignment marks to be measured within alimited timeframe. The metrology sensors 20 may be spaced with respectto each other, such that the substrate 12 is located underneath them.Having three sensors 20 allows three measurements from three separatelow NA alignment marks to be taken at the same time. This may allow, forexample, 200 low NA alignment marks to be measured during the scanningof the substrate 12 by the metrology sensors 20 (e.g., by movement ofthe substrate 12 relative to the metrology sensors 20).

In other examples, one of the metrology sensors 20 may be an IR(infrared) sensor. An example of such an IR sensor is given in theEuropean patent application no. EP 17181375.1, which is incorporatedherein in its entirety by reference. The IR sensor may be configured todetect radiation having wavelengths that pass through an opaque layer ofthe substrate 12. This allows measurements to be taken of features (e.g.alignment marks) which are not possible using other metrology sensors20. In other examples, one of the metrology sensors may be any suitablesensor that allows a measurement to be taken from below an upper (e.g.opaque) layer of the substrate 12.

FIG. 7 represents a flow of a method according to an embodiment. Firstposition data for a plurality of (e.g., low NA) alignment marks isobtained (e.g., using a low- and/or high-NA sensor) (step 100). Thenasymmetry data of the (e.g., low NA) alignment marks is obtained; e.g.,using a high-NA sensor (step 102).

Step 104 comprises determining a substrate grid based on the positiondata and optionally the asymmetry data. The substrate 12 may have beendeformed from the processes it has been subjected to, and the substrategrid will show where these deformations are located. However, the low NAalignment marks may also have been deformed from the processes that thesubstrate 12 has been subjected to, i.e. the low NA alignment marks mayhave an asymmetry. This means that the position data from the low NAalignment marks may not be accurate and thus the substrate grid showingthe deformation may not be accurate. The asymmetry data from the low NAalignment marks may be used to correct for the asymmetry of the low NAalignment marks and thus provide an improved substrate grid (anasymmetry-corrected substrate grid). In other words, the asymmetry datamay be used to reduce or minimize the effect of the asymmetry of the lowNA alignment marks on the substrate grid. As such, theasymmetry-corrected substrate grid may comprise a more accuratesubstrate grid describing a deformation of the substrate 12, asdetermined from the position and asymmetry data.

At step 106, this asymmetry-corrected substrate grid is sent to thelithographic apparatus. Alternatively, at step 106 (or in addition) theuncorrected substrate grid (a substrate grid based on positionmeasurements only) may be determined and sent together with theasymmetry data to the lithographic apparatus.

As such, once the substrate grid has been determined and the asymmetrydata has been measured, the substrate 12 is passed to the lithographicapparatus LA as shown by arrow 22 in FIG. 4, along with theasymmetry-corrected substrate grid and/or the uncorrected substrate gridand the asymmetry data.

Where the substrate grid forwarded at step 106 is uncorrected forasymmetry, the asymmetry-corrected substrate grid may be determinedby/within the lithographic apparatus. This should be done before thesubstrate grid is used to control a subsequent step 108 of exposing thesubstrate to fabricate one or more features on the substrate.

FIG. 5 shows a lithographic apparatus having a LA first stage 24 and anLA second stage 26. In the LA first stage 24, a LA alignment sensor 28(hereinafter referred to as LA alignment sensor 28) is provided. The LAalignment sensor 28 may be a low NA sensor. In the LA second stage 26,an exposure apparatus 30 is provided. After exposure of the substratethe substrate may be transferred to a metrology apparatus (for examplean overlay measurement apparatus), as indicated by the arrow 32.

In conjunction with, e.g., position measurement system IF shown in FIG.1, the LA alignment sensor 28 is used to align the substrate 12 in thelithographic apparatus LA and further determine a substrate grid asmeasured within the lithographic apparatus (the “lithographic apparatussubstrate grid”).

Where the substrate grid as determined by the FFMC 10 is anasymmetry-corrected substrate grid, as described, this already accuratesubstrate grid may be mapped to the substrate grid as determined by analignment process in the lithographic apparatus (e.g., with thelithographic apparatus substrate grid). The substrate grid obtained bymapping the asymmetry-corrected substrate grid to the lithographicapparatus substrate grid may be used for controlling at least part of anexposure process to fabricate one or more features on the substrate 12(step 108 of FIG. 7). This may comprise aligning the substrate 12 in thelithographic apparatus LA based on the substrate grid and asymmetry data(step 110 of FIG. 7), for example based on asymmetry-corrected substrategrid.

In case the substrate grid as determined by the FFMC 10 has not yet beencorrected for asymmetry, the lithographic apparatus LA may use thesubstrate grid and the asymmetry data to reduce or minimize the effectof the asymmetry of the alignment marks on the lithographic apparatussubstrate grid and the substrate grid as measured by the FFMC 10. Ageneral method of using asymmetry data to improve accuracy of asubstrate grid based on measurement of marks is disclosed in U.S. Pat.No. 9,778,025, which is incorporated herein in its entirety byreference.

Measuring the position data and asymmetry data of the low NA alignmentmarks prior to the substrate 12 being passed to the lithographicapparatus LA allows more alignment marks to be measured than can becarried out in the lithographic apparatus LA. This is because there is atime limit set for measurement in production (during exposure), e.g. toallow maximum throughput of substrates 12 in the lithographic apparatusLA. For example, using the FFMC 10 allows e.g. 200-600 measurements tobe taken of the x-y position of the alignment marks which compares toonly about 40 different alignment mark measurements that can be taken inthe lithographic apparatus LA. The more marks that are measured, thebetter the accuracy of the determined substrate grid (representative ofa certain substrate deformation).

In other examples, the metrology sensors 20 may measure position data ofsecond features, which in an exemplary case may comprise metrologytargets. The metrology targets may be an ensemble of composite gratings,formed by a lithographic process (e.g., either in resist (more common)or after an etch process). Typically the pitch and line-width of thestructures in the gratings is selected based on the measurement optics(in particular the NA of the optics) to be able to capture diffractionorders coming from the metrology targets.

The diffracted radiation may be used to determine positional shiftsbetween two layers (also referred to as ‘overlay’) or may be used toreconstruct at least part of the geometry of the grating as produced bythe lithographic process. This reconstruction may be used to provideguidance of the quality of the lithographic process and may be used tocontrol at least part of the lithographic process. The metrology targetsmay be overlay targets. The overlay targets may be a diffraction gratingwith a particular pitch, and the overlay targets are usually smallerthan alignment marks. In other examples, the metrology sensors 20 maymeasure asymmetry data of the second features. The function of thesecond features (e.g., metrology targets) is in this document mostlyassociated with alignment purposes and since they need to be measuredusing a high NA sensor (the metrology sensors), they are referred to inthis document as high NA alignment marks. The high NA alignment marksare typically densely distributed across the substrate and specificallydistributed densely enough to allow characterization of the substratedeformation at a spatial scale comparable to individual exposure fields.The high NA alignment marks are for example positioned in the scribelanes between the dies within the exposure fields and/or the scribelanes between the exposure fields. In other words, measurement ofpositions associated with high NA alignment marks may allow a higherresolution (typically intra-field) characterization of the substrategrid.

The intra-field high NA alignment mark positions may be used todetermine control targeted to optimize or improve exposure of individualfields across the substrate. The intra-field metrology data may beassociated with the same layer of the substrate 12 as to which overlayis measured (lower grating of an overlay mark distributed across twolayers). The intra-field data may be sampled at high spatial frequencyacross the substrate 12. The alignment data (of all fields) may beanalyzed for substrate 12 and alignment mark (either low NA or high NAalignment mark) deformation. The positions of the intra-field high NAalignment marks may further be added to the substrate grid (initiallybased on the low NA alignment mark position measurements).

Effectively the positions of the intra-field high NA alignment marks asmeasured in the FFMC 10 are sent to the exposure apparatus 30. Theadditional data will improve the characterization ofsubstrate-to-substrate variations of the substrate grid and the spatialaccuracy of corrections performed during exposure using the lithographicapparatus. This will decrease the substrate-to-substrate variations of,for example, overlay quality as measured after exposure. All existingfeedback, feedforward and Advanced Process Control loops may stay inplace.

Asymmetry measurements can be taken from both the low NA alignment marksand the high NA alignment marks. The high NA alignment marks generallyhave a smaller pitch than the low NA alignment marks. The impact ofprocessing (for example chemical mechanical planarization (CMP)) on theasymmetry of both mark types is normally not identical; typicallyasymmetry of smaller pitch marks is less sensitive to processing thanasymmetry associated with larger pitch marks. Hence dedicated asymmetrymeasurements for both low NA and high NA alignment marks are typicallyused. However, in embodiments described below, this requirement may bemitigated or obviated.

In case both low NA alignment mark position data and high NA alignmentmark positions are available they may be individually corrected usingthe available asymmetry data and, after correction, be merged to definea high resolution substrate grid. As described before, thishigh-resolution substrate grid may be mapped to the lithographicapparatus substrate grid and subsequently used to control thelithographic apparatus LA during exposure of the substrate.

Previously there was substrate-to-substrate variation in the substrategrid because there was insufficient high-frequency spatial information(for example information having a spatial resolution of 1-10 mm). Theconcepts described herein provide a high-resolution substrate grid,hence mitigating the problem considerably. Further, there was nointra-field substrate-to-substrate correction capability. Thehigh-resolution substrate grid includes intra-field information andhence also this problem is considerably reduced. The alignmentmeasurements done within the lithographic apparatus were limited toposition measurements of low NA alignment marks, hence not directlycorresponding to positions of metrology targets used for overlay controlof the lithographic apparatus. It is proposed to include positionmeasurements of high NA alignment targets, being typically identical totargets used for overlay measurements. Hence an embodiment of theinvention provides an efficient way of controlling the lithographicapparatus based on alignment measurements expected to demonstrate ahigher effectiveness in reducing substrate to substrate variations ofmeasured overlay.

Typically a calibration is executed in order to match the FFMC 10alignment results with those of the alignment results of thelithographic apparatus LA. This calibration covers mainly sensor tosensor and substrate table WT to substrate table WT differences betweenthe FFMC 10 and the lithographic apparatus LA. The FFMC 10 alignmentdata is mapped to the alignment data of the lithographic apparatus LA.The alignment data may be taken from alignment markers on a referencesubstrate followed by storage of calibration parameters, oralternatively the alignment data may be taken from alignment markers foreach substrate passing the FFMC and the lithographic apparatus.

One of the metrology sensors 20 may be calibrated to the LA alignmentsensor 28 in order to correlate position measurements made by themetrology sensor 20 to the position measurements made by the LAalignment sensor 28. The low NA alignment mark positions would then needto be measured by both the FFMC 10 and the lithographic apparatus LA.Alternatively the metrology sensor 20 may be calibrated towards thealignment sensor 18 on the first stage of the FFMC 10. When thealignment sensor 18 is calibrated towards the LA alignment sensor 28within the lithographic apparatus also the metrology sensor 20 will thenbe calibrated towards the alignment sensor 28.

At least a selection of position data associated with the plurality ofhigh NA alignment marks may be calibrated towards position dataassociated with the plurality of low NA alignment marks. At least aselection of the position data may be calibrated towards furtherposition data obtained during the aligning of the substrate 12 in thelithographic apparatus LA. In general it is desired that positionsassociated with a common set of low NA alignment marks are measured bythe alignment sensor 18 on the first stage of the FFMC 10, the alignmentsensor of the lithographic apparatus LA and the metrology sensors 20 onthe second stage of the FFMC 10. Further position data obtained duringthe aligning of the substrate 12 in the lithographic apparatus LA may becalibrated towards at least a selection of the position data.

FIG. 6 shows an overlay measurement apparatus 36 which includes a stage34. Overlay is a measure of how accurately the features on one layer ofthe substrate 12 are fabricated on top of underlying features of anotherlayer of the substrate 12.

Using the asymmetry data of the high NA alignment marks allows betteroverlay measurements. This is because the asymmetry of the high NAalignment marks is likely to be the same as the asymmetry of the overlaytargets. Thus, the asymmetry data of the high NA alignment marks may bepassed to the metrology apparatus 36 for measuring overlay (step 112 ofFIG. 7). At step 114 of FIG. 7, overlay data can be determined based onan overlay measurement and the asymmetry data from step 112. Using theasymmetry data of the high NA alignment marks provides an increase inaccuracy of the overlay measurement as the positions of the overlaytargets can be determined more accurately. An example of how asymmetrydata is used to improve overlay measurement accuracy is disclosed inU.S. Pat. No. 9,134,256, which is incorporated herein in its entirety byreference.

The asymmetry data is used to reduce or minimize the effect of theasymmetry of the overlay targets on the overlay measurement as done inthe overlay measurement apparatus 36. An improved overlay measurementprovides increased feedback accuracy to the lithographic apparatus LA.

There are two main optical OV (overlay) metrology concepts: image basedOV metrology (IBO) and diffraction based OV metrology (DBO). In the caseof IBO an OV target is built up of X and Y resist gratings that arespatially separated from X and Y gratings in the product layer. DBO usesgratings in an upper and lower layer but, in contrast to IBO, thegratings are not placed alongside each other but on top of each other.If the gratings are perfectly aligned (=zero overlay error) they form asymmetric composite grating with a symmetric scattering property.However, a small misalignment (overlay error≠0) creates an asymmetriccomposite grating which creates an asymmetry in the intensity ofdiffracted radiation.

Previously there was substrate-to-substrate variation in the metrologymeasurement due to a variation in Bottom Grating Asymmetry (BGA) persubstrate. The BGA may now be measured prior to overlay measurementusing the metrology sensors 20 within the FFMC. The determination of theoverlay data may now use the asymmetry data associated with the lowergrating part of the overlay target (as provided by the metrology sensors20) leading to a smaller measured substrate to substrate variation ofthe overlay data.

Although the above description has shown that the FFMC 10 is locatedoutside the lithographic apparatus LA, in other examples, the FFMC 10may be located in the lithographic apparatus LA or may be part of thelithographic apparatus LA. Alignment in the lithographic apparatus LAmay be considered to be similar to pre-exposure metrology. The alignmentsystem in the lithographic apparatus LA may be capable of both measuringposition of low NA alignment marks and high NA alignment marks andasymmetry of marks, e.g. by using a high NA sensor.

A further embodiment will now be described which determines a processdistortion (e.g., a process asymmetry) in first features such asalignment marks (e.g., low NA alignment marks), and uses this to correctmeasurement of a process parameter such as overlay for processdistortion (e.g., process asymmetry) in second features (e.g., metrologytargets such as overlay targets used to measure overlay). The processasymmetry is the asymmetry induced by processing effects, such as afloor tilt or side wall angle asymmetry. This should be the onlyasymmetry in alignment marks; however overlay targets will typicallycomprise other asymmetries, induced by the overlay being measured andany deliberate bias, if present. In this embodiment the asymmetry in thealignment marks may be measured using FFMC 10. However, in anotherembodiment which will be described more fully, the asymmetry in thealignment marks is measured using the lithographic apparatus LA (e.g.,as part of a standard alignment process using LA alignment sensor 28 toalign the substrate for exposure). As such, this embodiment may or maynot comprise use of an FFMC 10.

Currently, on a lithographic apparatus, there may be no feed forward ofalignment mark asymmetry from the alignment mark measurement (e.g., byLA alignment sensor 28) to an overlay measurement performed on ametrology device (e.g., a scatterometry device), such as overlaymeasurement apparatus 36. Since the physics of the alignment sensor andmetrology sensor are typically similar, i.e., based on interference ofthe +/−1 diffraction orders from a grating, it is reasonable to supposethat there is some correlation in the alignment position measurement tothe overlay measurement, from mark asymmetry in the alignment mark andoverlay target, respectively.

As already mentioned, because LA alignment sensor 28 typically measurestargets (e.g., low NA alignment marks) which have a different (e.g.,larger) pitch than the targets (e.g., high NA overlay targets) measuredby overlay measurement apparatus 36, the effect of asymmetry onalignment measurement is different than the effect of asymmetry onoverlay measurement. Asymmetry in the alignment mark causes an alignmenterror which, in turn causes an error in the overlay measurement which isadditional to any error in the overlay measurement resultant fromprocess (non-overlay/bias) asymmetry in the overlay target.

Process asymmetry in an alignment mark is typically different fromprocess asymmetry in an overlay target due to loading effects.Furthermore, one or both of these process asymmetries may be differentto in-product process asymmetries. Sometimes, there is no processasymmetry in-product, but there is process asymmetry in alignment and inoverlay with the asymmetry being different for the two. In addition, thealignment error resultant from alignment mark asymmetry will typicallybe different than an overlay measurement error due to the overlay targetprocess asymmetry, as overlay measurements and alignment measurementstypically have different sensitivities to asymmetry in the respectivefeature being measured. Consequently, there is a complex coupling ofoverlay measurement error resultant from alignment mark asymmetry andresultant from overlay target process asymmetry. A method for decouplingthese effects will be described.

The measured alignment grid, ā_(meas) is a function ƒ of the truealignment grid ā_(true) (i.e., the alignment grid without error due tomark asymmetry) and an alignment asymmetry component ā_(asym) (errorcomponent) of the measured alignment grid, resultant from alignment markasymmetry; i.e.,ā _(meas) =f(ā _(true) ,ā _(asym))

Measured overlay ō_(meas) is a function g of the true overlay ō_(true)(i.e., the overlay without error due to process asymmetry in thetarget), the measured alignment grid, ā_(meas) and an overlay asymmetrycomponent ō_(asym) (error component) of the measured overlay, resultantfrom overlay target process asymmetry (asymmetry other than that due tooverlay including any bias); i.e.,ō _(meas) =g(ō _(true) ,ō _(asym) ,ā _(meas))ō _(meas) =g(ō _(true) ,ō _(asym) ,f(ā _(true) ,ā _(asym)))

The aim of this embodiment is to determine true overlay ō_(true) frommeasured overlay ō_(meas).

It has been observed that align position deviation APD, and morespecifically, color-to-color align position deviation (hereafter C2CAPD), is a quality metric of the alignment marks which is correlatedwith the overlay asymmetry component ō_(asym). APD and C2C APD are eacha measure of the asymmetry in the alignment mark. For example, this datamay be measured using APD techniques or otherwise. APD techniques aredescribed in U.S. Pat. No. 8,982,347 and PCT Patent ApplicationPublication No. WO 2018/033499, each of which is incorporated herein itsentirety by reference. C2C APD is the difference in align position fortwo wavelengths. More specifically, some alignment sensors willtypically illuminate the alignment marks with a range of differentwavelengths of radiation (colors), and perform a color-to-coloranalysis, such as C2C APD, to correct for alignment errors caused byalignment mark asymmetry. Comparison between signals obtained withdifferent colors can indicate and quantify the presence of markasymmetry.

It is proposed, therefore, that C2C APD be used to correct overlaymeasurements; for example as a feed-forward correction from an alignmentmeasurement. Based on the C2C APD data, a correction for the effect ofprocess asymmetry in an overlay target may be fed forward to an overlaymeasurement apparatus (or processing apparatus which processes overlaydata from an overlay measurement apparatus) to correct an overlaymeasurement. In this way, it becomes possible to decouple the effects ofoverlay target process asymmetry and alignment mark asymmetry on theoverlay measurement.

FIG. 8 is a flow diagram describing a method according to thisembodiment. The measured alignment grid ā_(meas) 800 is obtained.Alignment grid ā_(meas) is a function of true alignment ā_(true) andalignment asymmetry component ā_(asym). C2C APD data 810 is obtainedcomprising the difference between the measured alignment grid, ā_(meas)for at least two different wavelengths, e.g., λ_(i) and λ_(j) (for agiven polarization pol); i.e., ā_(meas)(λ_(i), pol)−ā_(meas)(λ_(j),pol). The overlay asymmetry component ō_(asym) is determined from theC2C APD data 810 and measured overlay data 820 ō_(meas), via theapplication of a suitable function h, where ō_(asym)=h[ā_(meas)(λ_(i),pol), ā_(meas)(λ_(j), pol)]. The measured alignment grid ā_(meas) 800may be obtained from the same measurement as that used to derive the C2CAPD data 810.

The true overlay ō_(true) 830 (corrected for the effect of overlayasymmetry component ō_(asym)) can then be calculated from measuredoverlay data 820 ō_(meas), measured alignment grid 800 ā_(meas) andoverlay asymmetry component ō_(asym) via the application of a suitablefunction g, where ō_(meas)=g(ō_(true), ō_(asym), ā_(meas)). In this waya feed forward overlay asymmetry correction is determined form thealignment data (more specifically C2C APD data 810) and applied tocorrect the overlay measurement.

A training or modeling phase should be performed to determine functionsh and g. These functions may be determined through machine learningtraining and/or a suitable model. The model may be constructed, forexample, by correlating the C2C APD data to an expected deformationcharacteristic (asymmetry) of the overlay targets. A suitable modellingtechnique may comprise that known as “Design for Control”, abbreviatedas D4C, which is described in U.S. Patent Application Publication No. US2016-0140267, which is incorporated herein in its entirety by reference.In a D4C method, individual steps of a lithography process are modeledinto a single process sequence to simulate the physical substrateprocessing. That process sequence drives the creation of the devicegeometry as a whole, rather than “building” the device geometryelement-by-element. This is different from conventional approaches thatuse purely graphical volume elements in a three-dimensional schematiceditor to build metrology targets. The method enables the automaticgeneration of robust metrology targets in the simulation domain, whichcan accommodate a variety of lithography processes and processperturbations.

As such, a calibration stage may comprise simulating (e.g., using theaforementioned D4C modelling technique or other suitable method) variousalignment marks and overlay targets (e.g., having various differentdesign parameters, pitches etc.) which have been subject to the same(simulated) processing and manufacturing steps on one or more commonsubstrates (e.g., formed by common simulated deposition and/or exposureand/or etch processes etc.). The simulation may vary the effects of thevarious steps, and/or vary different degrees and/or types of asymmetryin the marks and targets. The simulated alignment marks and overlaytargets, and corresponding simulated measurement data (e.g., data fromsimulated alignment measurements of the simulated alignment marks toobtain simulated C2C APD data and simulated overlay measurements of thesimulated overlay targets to obtain simulated overlay data) can then beused as training data. Alternatively and/or in addition, the trainingdata may comprise actual measurements on physical features (i.e.,machine learning on actual measured data).

A machine learning technique can then be used to determine functions gand h from the relevant parameters of the simulated (and/or measured)alignment marks and overlay targets and simulated (and/or measured)measurement data (e.g., from the simulated measurement data and knowntrue values and asymmetries from the simulated features). For example,the training data could train a neural network or Boolean network todetermine functions g and h from the training data (this training may bemeasurement recipe specific or otherwise).

Once these functions are determined they can be used in a manufacturingsetting to correct an overlay measurement (e.g., a single wavelengthoverlay measurement) for the effect of unwanted process asymmetry; i.e.,asymmetry from effects other than overlay (including any deliberatebiases) based on a feed-forward correction determined from the processasymmetry of alignment measurements (e.g., more specifically C2C APDmeasurements).

Alternatively, a suitable machine learning network (e.g., neural networkor Boolean network) could be trained, using the training data, todetermine true overlay ō_(true) directly from the other measurementdata, e.g., C2C APD measurement data ā_(meas)(λ_(i),pol)−ā_(meas)(λ_(j), pol) 810, alignment asymmetry component ā_(asym)(e.g., as determined from alignment mark asymmetry data 800) andmeasured overlay ō_(meas) 820. In such an embodiment, the trainednetwork could then infer the true overlay ō_(true) directly from themeasurements. Such a training embodiment may work better and/or requireusing actual measurement data rather than simulated data.

It is mentioned above that the measured overlay may comprise singlewavelength overlay data. Presently, overlay may be measured using an (atleast) three wavelength method (called HMQ) to remove overlay markasymmetry from the overlay measurement. Further details of HMQ can befound in PCT Patent Application Publication No. WO 2015/018625, which isincorporated herein in its entirety by reference. Very briefly, HMQcomprises measuring a target comprising a +d biased sub-target and a −dbiased sub-target with multiple wavelengths, and fitting a line througha plot of asymmetry measurements for the first sub-target againstasymmetry measurements for the second sub-target structure, the linearregression model not necessarily being fitted through the origin. Theoffset from the origin of this line (also referred to as distance toorigin DTO) is representative of the target process asymmetry.

However, the MAM (move-acquire-measure) time for the total overlaymeasurement tends to be directly proportional to the number ofwavelengths measured (e.g., the MAM for a three wavelength measurementis approximately three times that of a single wavelength measurement).Because of the long MAM time of present overlay measurements, it istypical to only measure overlay on only one substrate per lot. Theproposed method could be used to significantly reduce the MAM(move-acquire-measure) time for (HMQ) overlay as three wavelengths maynot be needed, thereby increasing measurement speed and thereforethroughput. As such, a measurement may comprise only a singleillumination characteristic measurement (e.g., only a single wavelengthmeasurement). Alternatively, or in addition, the proposed method may beused to drive improvements in overlay and yield, by allowing moreoverlay targets to be measured accurately.

The method of this embodiment may further improve the accuracy of thecurrent HMQ method. Typically, only one substrate per lot is sampled inan overlay measurement without reducing the cluster throughput. Thealignment position measurement, which is measured per substrate, can beused on the overlay measurement to determine whether more substrates perlot need to be sampled or if different points per substrate should bemeasured.

More specifically, it is known that C2C APD is correlated with overlayasymmetry component ō_(asym), and therefore will be correlated with theHMQ DTO. Using the HMQ methodology, an acceptable range for the DTO canbe determined (e.g., it can be compared to a threshold DTO,DTO_(thres)). If it is determined that the DTO is outside of theacceptable range (e.g., abs(DTO)>DTO_(thres)), this would indicate thatthere is a lot of process asymmetry in the overlay mark. The value forthe threshold DTO_(thres) can be determined based on a case-by-casebasis. There is likely to be a correlation between the thresholdDTO_(thres) and yield. As a next step, the function h is determined (asalready described). Additionally, a second threshold, this time for theC2C APD is determined (alignment asymmetry threshold C2C APD_(thres)),where C2C APD_(thres) is a function of DTO_(thres). This function can bedetermined by h. Therefore, if it is determined that C2C APD is greaterthan C2C APD_(thres), the substrate can be flagged as having largeasymmetry which will most likely result in large overlay error. As such,this can be seen to be a classification machine learning method, andalignment measurement can be used as a process monitor (e.g., in spec orout of spec) for overlay measurement. This should improve yield.

As explained herein, alignment measurement or overlay measurement istypically based on detecting diffraction orders scattered by a metrologystructure. Examples of such metrology structures are, for alignment,alignment marks or, for overlay, overlay marks (comprising at least abottom grating). The energy comprised within the diffraction orderdepends on the diffraction efficiency of the metrology structure. Thisdiffraction efficiency partially depends on the pitch and duty cycle ofthe metrology structure(s) and partially depends on one or more stackcharacteristics in which the metrology structure is embedded, whereinthe stack refers to the one or more layers of material on the substrateand in which the metrology structure is formed.

The mentioned metrology system commonly derives a property of interestassociated with the metrology structure based on a metric havingmeasured values of the diffraction order intensities (or energies) as aninput. For example, overlay measurement can be based on calculation of adifference in intensity between a first and a minus first diffractionorder scattered by the overlay mark. In the case of an overlaymeasurement the metric is hence a difference between two diffractionorder intensities. Typically the measurement using the overlay metrologytool or alignment measurement system is based on signals provided by asensor positioned in or near a pupil plane, e.g. the diffraction ordersare spatially separated at the plane in which the sensor surface lies.

To facilitate accurate measurement of individual diffraction orders itmay be necessary to configure the illumination pupil of the measurementradiation source used to illuminate the metrology structure. Theillumination pupil may be configured such that there is no, or verylimited, overlap between for example the first and minus firstdiffraction order and between the first diffraction orders and the zeroorder.

The difference metric scales with the overall diffraction efficiency ofthe metrology structure, which is largely determined by one or moreproperties of the stack, and specifically the interaction of the stackand a property (e.g., wavelength, polarization mode, etc.) of theradiation used to illuminate the metrology structure. In case of a smalldiffraction efficiency the signal generated by the metrology system maybe too small to enable accurate determination of the difference metric.

A sufficiently large diffraction efficiency may be achieved by selectingan appropriate wavelength and/or polarization mode of the radiation usedfor illuminating the metrology structure. For example a measurement ofthe intensity of a first diffraction order may be performed for aplurality of wavelengths and/or polarization modes. In another example asum metric of the intensities of the positive first and minus firstorder may be measured across a range of wavelengths and/or polarizationmodes, after which a wavelength and/or polarization mode may be selectedbased on an optimum sum metric associated with the intensities comprisedwithin the positive first and minus first order. The sum metric may bedetermined based on asymmetry data (as disclosed herein) comprising theintensities of the diffraction orders as measured at a plurality ofwavelengths and/or polarization modes. Also the asymmetry data maycomprise intensity data of diffraction orders scattered by a pluralityof features, for example by alignment marks and overlay (bottom grating)marks.

Once an optimal one or more (e.g., range of) wavelengths and/orpolarization modes has been determined based on the sum metric, a moreoptimal configuration of an overlay metrology tool and/or alignmentmeasurement system may be achieved. For example an optimum wavelengthand polarization mode determined by optimizing the sum metric asmeasured by the overlay metrology tool may be communicated to a computersystem configured to optimize the settings of a metrology tool, such asan alignment measurement system or a scatterometer (for exampleconfigured to measure overlay of the device manufacturing process). Thecomputer system may subsequently provide an optimized alignmentmeasurement recipe yielding a more accurate position measurement of thealignment mark(s).

The functional dependency of the sum metric on the wavelength and/orpolarization mode of the radiation further may be used to monitor theevolution of one or more stack properties due to change in one or moreprocessing steps (e.g. performed by one or more CMP tools, one or moredeposition tools or one or more etch tools). When, for example, a CMPtool used in the process drifts in time the thickness of one or morelayers comprised within the stack will likely vary. As a result anoptimum wavelength and/or polarization mode of the radiation in view ofa maximum sum metric is also likely to change as there is a strongcoupling between a stack characteristic (e.g. thickness of one or morelayers) and the diffraction efficiency of the metrology structure. Henceboth the value of the sum metric and its associated optimum wavelengthand/or polarization mode of the radiation may be used for monitoring ofone or more processing steps applied in manufacturing devices.

In many cases knowledge of the value of the sum metric and/or theoptimal wavelength and/or polarization mode may be linked to a preciselydetermined value of one or more stack parameters. For example a modeledrelation between a stack thickness and a diffraction efficiency of themetrology structure may be available. The modeled relation may be usedto translate an observed change in the sum metric to a predicted changeof the stack thickness. The predicted change in the stack thickness maybe used for process control purposes, for example applying a correctivecontrol action to a deposition tool used in applying a layer comprisedwithin the stack and/or a CMP tool used in polishing a layer comprisedwithin the stack. Hence apart from process monitoring, process controlalso may be based on knowledge of the sum metric and/or an opticalproperty (e.g., wavelength and/or polarization mode) of the radiationused in optimizing the sum metric.

It is recognized that the sum metric is a desirable metric to enable theabove mentioned configuring of a metrology system, monitoring of adevice manufacturing process or controlling a device manufacturingprocess. However, in general, knowledge of an integrated intensity orintegrated energy of at least a Nth diffraction and a −Nth diffractionorder may be sufficient, the integrated intensities or energiestypically being added to yield the sum metric. But also a weightedsummation may be considered in case one of the diffraction orders isknown to be less relevant than the other diffraction order.

The sum metric also may be used as a process parameter indicative of aperformance (or quality) parameter such as overlay, CD or focus. In thiscase a model may be trained correlating the sum metric to measuredperformance parameter data. The trained model may then be used topredict the performance parameter(s) based on the sum metric and the summetric may further be used in the context of a virtual/hybrid metrologysystem, the sum metric for example being used to upsample theperformance parameter data (as obtained by direct measurement of theperformance parameter for example). In an embodiment a model is trainedwith sum metric and performance parameter data, and the model is thensubsequently used to provide dense performance parameter data across asubstrate based on knowledge of the sum metric associated with thesubstrate.

Further embodiments are disclosed in the list of numbered clauses below:

1. A method for determining a substrate grid describing a deformation ofa substrate prior to exposure of the substrate in a lithographicapparatus configured to fabricate one or more features on the substrate,the method comprising:

obtaining position data for a plurality of first features and/or aplurality of second features on the substrate;

obtaining asymmetry data for at least a feature of the plurality offirst features and/or the plurality of second features;

determining the substrate grid based on at least the position data; and

passing the substrate grid and asymmetry data to the lithographicapparatus for controlling at least part of an exposure process tofabricate one or more features on the substrate.

2. The method of clause 1, wherein the determining of the substrate gridis additionally based on the asymmetry data, such that the step ofpassing the substrate grid and asymmetry data comprises passing anasymmetry corrected substrate grid.

3. The method of clause 1, wherein the plurality of first featurescomprises alignment marks.

4. The method of clause 3, further comprising obtaining the positiondata for the alignment marks using a low numerical aperture (NA) opticalsystem comprising a sensor having a NA of less than or equal to 0.9.

5. The method of any of clauses 1-4, wherein the plurality of secondfeatures comprises metrology targets and, optionally, overlay targets.

6. The method of clause 5, wherein the plurality of first featurescomprises alignment marks and further comprising obtaining the asymmetrydata for the alignment marks and/or the metrology targets using a highNA optical system comprising a sensor having a NA of greater than 0.9.7. The method of clause 6, wherein the high NA optical system comprisesa plurality of spatially distributed high NA sensors.8. The method of any of clauses 1-7, further comprising calibrating atleast a selection of position data associated with the plurality ofsecond features towards position data associated with the plurality offirst features.9. The method of clause 8, further comprising calibrating at least aselection of the position data towards further position data obtainedduring the aligning of the substrate in the lithographic apparatus.10. The method of clause 8, further comprising calibrating furtherposition data obtained during the aligning of the substrate in thelithographic apparatus towards at least a selection of the positiondata.11. The method of any of clauses 1-10, further comprising aligning thesubstrate in the lithographic apparatus based on the substrate grid andthe asymmetry data.12. The method of any of clauses 1-11, further comprising controlling atleast part of an exposure process in the lithographic apparatus tofabricate one or more further features thereon based on the substrategrid and asymmetry data, wherein the first and/or second features arelocated on a first layer and the one or more further features arelocated on a second, higher layer.13. The method of any of clauses 1-12, further comprising passing theasymmetry data to a metrology apparatus for measuring overlay.14. The method of clause 13, further comprising determining overlay databased on the asymmetry data.15. A measurement apparatus for determining a substrate grid describinga deformation of a substrate prior to exposure of the substrate in alithographic apparatus configured to fabricate one or more features onthe substrate, the measurement apparatus comprising:

an optical system configured to obtain position data for a plurality offirst features and/or a plurality of second features on the substrate,

wherein the optical system is further configured to obtain asymmetrydata for at least a feature of the plurality of first features and/orthe plurality of second features, and

wherein the measurement apparatus is configured to determine thesubstrate grid based on the position data and pass the substrate gridand asymmetry data to the lithographic apparatus for controlling atleast part of an exposure process to fabricate one or more features onthe substrate.

16. The measurement apparatus of clause 15, wherein the plurality offirst features comprises alignment marks.

17. The measurement apparatus of clause 16, wherein the optical systemcomprises a low numerical aperture (NA) optical system comprising asensor having an NA of less than or equal to 0.9 which is configured toobtain the position data for the alignment marks.18. The measurement apparatus of any of clauses 15-17, wherein theplurality of second features comprises metrology targets and,optionally, overlay targets.19. The measurement apparatus of clause 18, wherein the plurality offirst features comprises alignment marks and wherein the optical systemcomprises a high NA optical system comprising a sensor having a NA ofgreater than 0.9 which is configured to obtain the asymmetry data forthe alignment marks and/or the metrology targets.20. The measurement apparatus of clause 19, wherein the high NA opticalsystem comprises a plurality of spatially distributed high NA sensors.21. The measurement apparatus of clause 20, wherein the plurality ofspatially distributed high NA sensors comprises at least one sensorconfigured to detect radiation having wavelengths that pass through anopaque layer of the substrate, optionally the at least one sensor is anIR (infrared sensor).22. The measurement apparatus of any of clauses 15-21, wherein themeasurement apparatus is configured to calibrate at least a selection ofposition data associated with the plurality of second features towardsposition data associated with the plurality of first features.23. A lithographic apparatus configured to use the substrate grid andasymmetry data of clause 1, wherein the lithographic apparatus isconfigured to align the substrate in the lithographic apparatus based onthe position data and/or the asymmetry data.24. The lithographic apparatus of clause 23, wherein the lithographicapparatus comprises a metrology apparatus configured to determineoverlay data based on the asymmetry data.25. The lithographic apparatus of clause 23, comprising the measurementapparatus of clause 15.26. A method for determining a value for a process parameter measurementerror obtained from measurement of a substrate subject to amanufacturing process and comprising a target having a processdistortion, the process parameter measurement error being a result ofthe process distortion, the method comprising:

obtaining alignment asymmetry data describing asymmetry in one or morealignment marks used for aligning the substrate;

obtaining a model correlating alignment asymmetry data to the processparameter measurement error; and

using the alignment asymmetry data and the model to obtain the value ofthe process parameter measurement error.

27. The method of clause 26, wherein the process parameter is overlay.

28. The method of clause 26 or clause 27, wherein the alignmentasymmetry data comprises a difference in a first measured position ofthe one or more alignment marks when measured using radiation with afirst characteristic and a second measured position of the one or morealignment marks when measured using radiation with a secondcharacteristic.29. The method of clause 28, wherein the characteristic which is variedbetween the first characteristic and the second characteristic iswavelength and/or polarization.30. The method of any of clauses 26 to 29, comprising determining acorrection for a measurement of the process parameter from the processparameter measurement error.31. The method of clause 30, wherein the measurement of the processparameter is based on a measurement of the target performed with asingle illumination characteristic measurement.32. The method of any of clauses 26 to 31, comprising performing acalibration stage to calibrate the model.33. The method of clause 32, wherein the calibration stage is performedusing simulated training data comprising simulated targets and simulatedalignment marks, and simulated measurement responses of the simulatedtargets and simulated alignment marks.34. The method of clause 33, wherein the calibration stage calibratesthe model, such that the model can characterize the process parametermeasurement error based on the alignment asymmetry data.35. The method of any of clauses 26 to 34, wherein the model comprises aneural network.36. The method of any of clauses 26 to 35, wherein the alignmentasymmetry data is further used to determine whether the manufacturingprocess is within specification.37. The method of clause 36, comprising determining an amendment to ametrology action for measuring the process parameter based on thedetermination as to whether the manufacturing process is withinspecification.38. The method of any of clauses 26 to 37, comprising reducing thenumber of acquisitions of a metrology action to measure the processparameter, wherein each acquisition is performed with a differentillumination characteristic.39. The method of any of clauses 26 to 37, comprising comparing thealignment asymmetry data to an alignment asymmetry threshold andcategorizing the substrate based on the comparison.40. The method of clause 39, wherein the categorizing the substratecomprises determining whether the substrate is within or outside ofspecification based on the comparison.41. A lithocell comprising a lithographic apparatus, a metrologyapparatus, and at least one processor operable to perform the method ofany of clauses 26-40.42. The lithocell of clause 41, wherein the lithographic apparatus isoperable to perform alignment measurement to obtain the alignmentasymmetry data; and the metrology apparatus is operable to measure theprocess parameter; wherein the at least one processor is operable tocorrect the measured process parameter using the value of the processparameter measurement error.43. A method for configuring a metrology tool used in a devicemanufacturing process, the method comprising: obtaining an integratedintensity or integrated energy comprised within a +Nth and −Nth order ofdiffraction scattered from a metrology structure applied to a layer on asubstrate illuminated at a plurality of wavelengths and/or polarizationmodes; and configuring the metrology tool based on the dependency of theintegrated intensity or integrated energy to the wavelength and/orpolarization mode, wherein the configuring provides at least selectionof a wavelength and/or polarization mode of a source of radiation withinor for the metrology tool.44. A method for monitoring a device manufacturing process, the methodcomprising: obtaining an integrated intensity or integrated energycomprised within a +Nth and −Nth order of diffraction scattered from ametrology structure applied to a layer on a substrate illuminated at aplurality of wavelengths and/or polarization modes; and monitoring themanufacturing process based on a property of the dependency of theintegrated intensity or integrated energy to the wavelength and/orpolarization mode.45. A method for controlling a device manufacturing process, the methodcomprising: obtaining an integrated intensity or integrated energycomprised within a +Nth and −Nth order of diffraction scattered from ametrology structure applied to a layer on a substrate illuminated at aplurality of wavelengths and/or polarization modes; and controlling aprocessing apparatus used in the manufacturing process based on aproperty of the layer derived from the dependency of the integratedintensity or integrated energy to the wavelength and/or polarizationmode.46. The method of any of clauses 43, 44 or 45, wherein the obtaining isbased on measurement data provided by a measurement device.47. The method of clause 46, wherein the measurement device comprises analignment system.48. The method of any of clauses 43 to 47, wherein the metrologystructure comprises an alignment mark.49. The method of clause 46, wherein the measurement device comprises ascatterometer.50. The method of any of clauses 43 to 49, wherein the metrologystructure comprises a bottom grating of an overlay mark.51. The method of clause 46, wherein the obtaining is based onmeasurement within a pupil plane of the measurement device.52. The method of clause 51, wherein the illuminating of the metrologystructure uses a radiation source having an illumination pupilconfigured to have no, or very limited, overlap between diffractionsorders and/or overlap between a diffraction order and the zero order.53. The method of any of clauses 43 to 52, wherein the obtaining furthercomprises summation of the integrated intensities and/or integratedenergies comprised within the +Nth and −Nth diffraction orders.54. The method of clause 1, wherein the asymmetry data further comprisesintensity data or energy data of individual diffraction orders scatteredfrom the first and/or second features illuminated at a plurality ofwavelengths and/or polarization modes.55. The method of clause 54, further comprising: obtaining an integratedintensity or integrated energy comprised within a +Nth and/or −Nth orderof diffraction at the plurality of wavelengths and/or polarization modesbased on the asymmetry data; and monitoring a device manufacturingprocess based on a property of the dependency of the integratedintensity or integrated energy to the wavelength and/or polarizationmode.56. The method of clause 54, further comprising: obtaining an integratedintensity or integrated energy comprised within a +Nth and/or −Nth orderof diffraction at the plurality of wavelengths and/or polarization modesbased on the asymmetry data; and controlling a processing apparatus usedin a device manufacturing process based on a property of the layerderived from the dependency of the integrated intensity or integratedenergy to the wavelength and/or polarization mode.57. The method of clause 54, further comprising: obtaining an integratedintensity or integrated energy comprised within a +Nth and/or −Nth orderof diffraction at the plurality of wavelengths and/or polarization modesbased on the asymmetry data; and configuring a metrology tool based onthe dependency of the integrated intensity or integrated energy to thewavelength and/or polarization mode, wherein the configuring provides atleast selection of a wavelength and/or polarization mode of a source ofradiation within or for the metrology tool.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications. Possible other applications include the manufactureof integrated optical systems, guidance and detection patterns formagnetic domain memories, flat-panel displays, liquid-crystal displays(LCDs), thin-film magnetic heads, etc.

Although specific reference may be made in this text to embodiments ofthe invention in the context of a lithographic apparatus, embodiments ofthe invention may be used in other apparatus. Embodiments of theinvention may form part of a patterning device (e.g., mask) inspectionapparatus, a metrology apparatus, or any apparatus that measures orprocesses an object such as a wafer (or other substrate) or mask (orother patterning device). These apparatus may be generally referred toas lithographic tools. Such a lithographic tool may use vacuumconditions or ambient (non-vacuum) conditions.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention, where the context allows, is notlimited to optical lithography and may be used in other applications,for example imprint lithography.

To the extent certain U.S. patents, U.S. patent applications, or othermaterials (e.g., articles) have been incorporated by reference, the textof such U.S. patents, U.S. patent applications, and other materials isonly incorporated by reference to the extent that no conflict existsbetween such material and the statements and drawings set forth herein.In the event of such conflict, any such conflicting text in suchincorporated by reference U.S. patents, U.S. patent applications, andother materials is specifically not incorporated by reference herein.

In block diagrams, illustrated components are depicted as discretefunctional blocks, but embodiments are not limited to systems in whichthe functionality described herein is organized as illustrated. Thefunctionality provided by each of the components may be provided bysoftware or hardware modules that are differently organized than ispresently depicted, for example such software or hardware may beintermingled, conjoined, replicated, broken up, distributed (e.g. withina data center or geographically), or otherwise differently organized.The functionality described herein may be provided by one or moreprocessors of one or more computers executing code stored on a tangible,non-transitory, machine readable medium. In some cases, third partycontent delivery networks may host some or all of the informationconveyed over networks, in which case, to the extent information (e.g.,content) is said to be supplied or otherwise provided, the informationmay be provided by sending instructions to retrieve that informationfrom a content delivery network.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The descriptions above are intended to beillustrative, not limiting. Thus it will be apparent to one skilled inthe art that modifications may be made to the invention as describedwithout departing from the scope of the claims set out below.

The invention claimed is:
 1. A method comprising: obtaining positiondata, measured outside of a lithographic apparatus configured tofabricate one or more features on a substrate, for a plurality of firstfeatures and/or a plurality of second features on the substrate;obtaining asymmetry data for at least a feature of the plurality offirst features and/or the plurality of second features; determining, bya hardware computer system, the substrate grid based on at least theposition data, the determined substrate grid describing deformationacross the substrate prior to exposure of the substrate in thelithographic apparatus; and passing the substrate grid and asymmetrydata to the lithographic apparatus for controlling at least part of anexposure process to fabricate one or more features on the substrate. 2.The method of claim 1, wherein the determining of the substrate grid isadditionally based on the asymmetry data, such that the passing thesubstrate grid and asymmetry data comprises passing an asymmetrycorrected substrate grid.
 3. The method of claim 1, wherein theplurality of first features comprises alignment marks.
 4. The method ofclaim 3, further comprising obtaining the position data for thealignment marks using a low numerical aperture (NA) optical system,comprising a sensor, having a NA of less than or equal to 0.9.
 5. Themethod of claim 1, wherein the plurality of second features comprisesmetrology targets.
 6. The method of claim 5, wherein the plurality offirst features comprises alignment marks and further comprisingobtaining the asymmetry data for the alignment marks and/or metrologytargets of the plurality of second features, using a high NA opticalsystem, comprising a sensor, having a NA of greater than 0.9.
 7. Themethod of claim 1, further comprising calibrating at least a selectionof position data associated with the plurality of second featurestowards position data associated with the plurality of first features.8. The method of claim 7, further comprising calibrating at least aselection of the position data towards further position data obtainedduring the aligning of the substrate in the lithographic apparatus. 9.The method of claim 1, further comprising passing the asymmetry data toa metrology apparatus for measuring overlay.
 10. The method of claim 1,wherein the asymmetry data further comprises intensity data or energydata of diffraction orders scattered from the first features and/orsecond features at a plurality of wavelengths and/or polarization modes.11. The method of claim 10, further comprising: obtaining, based on theasymmetry data, an integrated intensity or integrated energy comprisedwithin a +Nth and/or −Nth order of diffraction at the plurality ofwavelengths and/or polarization modes; and configuring a metrology toolbased on the dependency of the integrated intensity or integrated energyto the wavelength and/or polarization mode, wherein the configuringprovides at least selection of a wavelength and/or polarization mode ofa source of radiation within or for the metrology tool.
 12. Alithographic apparatus configured to use the substrate grid andasymmetry data of claim 1, wherein the lithographic apparatus isconfigured to align the substrate in the lithographic apparatus based onthe position data and/or the asymmetry data.
 13. A measurement apparatuscomprising: an optical system outside of a lithographic apparatusconfigured to fabricate one or more features on a substrate, the opticalsystem configured to obtain position data for a plurality of firstfeatures and/or a plurality of second features on the substrate, whereinthe optical system is further configured to obtain asymmetry data for atleast a feature of the plurality of first features and/or the pluralityof second features, and wherein the measurement apparatus is configuredto determine the substrate grid based on at least the position data, thedetermined substrate grid describing deformation across the substrateprior to exposure of the substrate in the lithographic apparatus, andpass the substrate grid and asymmetry data to the lithographic apparatusfor controlling at least part of an exposure process to fabricate one ormore features on the substrate.
 14. The measurement apparatus of claim13, wherein the optical system comprises a low numerical aperture (NA)optical system, comprising a sensor, having an NA of less than or equalto 0.9 which is configured to obtain the position data.
 15. Themeasurement apparatus of claim 14, further comprising a plurality ofspatially distributed high NA sensor systems.
 16. A computer programproduct comprising a non-transitory computer-readable medium havinginstructions, the instructions, upon execution by a computer system,configured to cause the computer system to at least: obtain positiondata, measured outside of a lithographic apparatus configured tofabricate one or more features on a substrate, for a plurality of firstfeatures and/or a plurality of second features on the substrate; obtainasymmetry data for at least a feature of the plurality of first featuresand/or the plurality of second features; determine a substrate gridbased on at least the position data, the determined substrate griddescribing deformation across the substrate prior to exposure of thesubstrate in the lithographic apparatus; and pass the substrate grid andasymmetry data toward the lithographic apparatus for controlling atleast part of an exposure process to fabricate one or more features onthe substrate.
 17. The computer program product of claim 16, wherein theinstructions configured to cause the computer system to determine thesubstrate grid are further configured to determine the substrate gridadditionally based on the asymmetry data, such that the passing of thesubstrate grid and asymmetry data comprises passing of an asymmetrycorrected substrate grid.
 18. The computer program product of claim 16,wherein the plurality of first features comprises alignment marks andwherein the instructions configured to cause the computer system toobtain the position data are further configured to obtain the positiondata for the alignment marks as measured using a low numerical aperture(NA) optical system, comprising a sensor, having a NA of less than orequal to 0.9.
 19. The computer program product of claim 16, wherein theinstructions are further configured to cause the computer system toobtain the asymmetry data for the plurality of second features asmeasured using a high NA optical system, comprising a sensor, having aNA of greater than 0.9.
 20. The computer program product of claim 16,wherein the instructions are further configured to cause the computersystem to calibrate at least a selection of position data associatedwith the plurality of second features towards position data associatedwith the plurality of first features.